High speed data transmission between complex electronic gear such as computers and their peripherals is commonly achieved through round multi-conductor cable assemblies. Inherent with high speed data transmission and multi-conductor cables are problems associated with electromagnetic interference. The interference between conductors can create "cross talk" and "noise", which can cause erroneous data transmission.
One type of cable used to reduce such effects is "twisted pair" cable, where two adjacent conductors or differential pairs are twisted, thereby reversing the lateral position of each conductor at each twist. The benefits of this configuration are two fold.
First, the position of the wires is constantly being reversed. Therefore any exterior magnetic or electric field has a generally uniform effect upon a given differential pair. This uniformity reduces the harmful effects of exterior electromagnetic fields and lessens the chances of erroneous data transmission.
Second, when current runs through a conductor, an electromagnetic field is generated. The field's orientation is dependent upon the direction of the current flow. For a given differential pair, electric currents generally flow in opposite directions. Therefore, the electromagnetic field radiating from each conductor generally has a canceling effect upon the other. This canceling effect can substantially suppress the radiated emissions from a given differential pair.
Flat flexible cable or circuitry (FFC) is another medium used for high speed data transmission between computers and their peripherals. A "pseudo-twisted" arrangement can be achieved with FFC by placing conductors of a given pair on opposite sides of the insulative carrier, with their paths slightly and oppositely offset with respect to a common nominal path locator, and then periodically reversing the offset at predetermined locations. An example of the "pseudo-twisted" flat flexible cable arrangement is shown in U.S. Pat. No. 3,761,842, dated Sep. 25, 1973. The characteristic impedance of flat flexible cable can affect the integrity of the signals travelling in the cable. One of the factors that influences the characteristic impedance of a given pair of conductors of a pseudo-twisted FFC is the centerline distance "L" (FIG. 3A--CASE 2) between conductors. In FIG. 3A, cable "B" includes pseudo-twisted conductors "P" that are positioned on opposite sides of a flexible substrate "d" with adhesive layers "e" and "c" interposed therebetween. The conductors are covered by protective films "a" and "g" with adhesive layers "b" and "f" interposed therebetween. The lateral distance "D" between the conductors on opposite sides of a prior art pseudo-twisted FFC varies along the length of the cable, as indicated by 0.2, 0.16 and 0 mm. If conductors "P" have constant widths and thicknesses along their lengths, the impedance "Z" decreases with a decrease in the conductor-to-conductor distance "D" (See FIG. 3B). Therefore, at every crossing point or "twist" of the conductors, the characteristic impedance decreases. Accordingly, the more twists per unit length of FFC, the lower the characteristic impedance for that given length.
U.S. Pat. No. 3,761,842 discloses a pseudo-twisted FFC having pairs of conductors, each pair having a differing numbers of twists per unit length. Characteristic impedance for a given pair of conductors in a pseudo-twisted FFC is also a function of the lateral width of the conductor paths. A reduction in the lateral width of the conductors increases the characteristic impedance for the given pair. U.S. Pat. No. 3,761,842 also teaches that if the conductor width at the crossing point is decreased, the characteristic impedance at that point is increased. This relationship allows the user to construct a cable where the characteristic impedance per unit length is independent of the number of twists per unit length. Accordingly, a cable with many twists per unit length would have the lateral width of the conductors at the crossing point reduced more than a cable with fewer twists per unit length. While the pseudo-twisted FFC of U.S. Pat. No. 3,761,842 has some advantages over the prior art, because of the abrupt reduction in the width of the conductors, the impedance will also change abruptly. This is undesirable in many applications such as in high speed data transmission where it is desirable to have a cable whose characteristic impedance is constant through out the length of the cable. The present invention is directed to providing a pseudo-twisted FFC having a characteristic impedance that is independent of the number of twists per unit length and substantially constant through out the length of the cable.
Additional prior art pseudo-twisted FFC cables are disclosed in Japanese Patent Publication Nos. 5-120928 and 6-111642.